The invention relates to a drafting unit for a spinning machine having several pairs of rollers which are each formed of a pressure roller provided with a covering and a drivable bottom roller provided with a profiling, an apron guide being provided in the last drafting zone in front of a pair of delivery rollers.
It is customary in practice to construct drafting units of the initially mentioned type such that all roller pairs, from the pair of feeding rollers to the pair of delivery rollers, essentially have the same diameter. In the case of commonly assigned pending application Ser. No. 07/379,646, filed on July 14, 1989 based on German Patent Application (P 38 25 068.3), which is not a prior publication, it is provided that the diameter of the pressure roller and of the bottom roller of the pair of delivery rollers is smaller than the diameter of the rollers of the preceding pair of rollers. As a result, it is achieved that the apron guide can extend closer to the nip line of the pair of delivery rollers so that the length is reduced in which the sliver is unguided. This improves the quality of the drawn sliver.
An object of the invention is to further improve a drafting unit of the initially mentioned type with respect to the drawing quality.
This object is achieved in that the pressure roller and the bottom roller of the pair of delivery rollers have a smaller diameter than the pressure roller and the bottom roller of the preceding pair of rollers, and that the thickness of the covering of the pressure roller of the pair of delivery rollers amounts to at least approximately 4 mm.
As a result, it is achieved that the apron guide may extend relatively closely to the nip line of the pair of delivery rollers so that, particularly at very high operating speeds, the danger is reduced of disturbances of the fiber position in the sliver. In addition, it is ensured that, as a result of the minimum thickness of the covering, the effect of the covering is maintained.
In a further development of the invention, it is provided that the bottom roller of the pair of delivery rollers has a profiling, the depth of which is no larger than approximately 0.4 mm. It is also provided that the width of the ribs of the profiling at the circumferential surface of the bottom roller of the pair of delivery rollers is no larger than approximately 0.4 mm. For a further improvement, it is also provided that the width of the recesses between ribs at the circumferential surface of the bottom roller of the pair of delivery rollers is no wider than approximately 0.6 mm. Thus, the profiling of the bottom roller, particularly a ribbing, is also adapted to the reduced diameter.
In order to achieve a bearing of the pressure roller of the pair of delivery rollers which is as stable as possible despite the smaller diameter, it is provided in a first embodiment that the pressure roller of the pair of delivery rollers is constructed as an over-mounted shaft which is provided with the covering at its end. In another embodiment of the invention, it is provided for the same purpose that the pressure roller of the pair of delivery rollers has a tube-shaped basic body which, in the area of the covering, is disposed on a shaft by means of a needle bearing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.